Motivated B.J. Penn credits Rory MacDonald for peak UFC on FOX 5 form

Dubiously, B.J. Penn estimated that he was 40 percent body fat when he began training for a fight with Rory MacDonald at UFC 152.

But five months later, he said he’s under 10 percent in advance of their rescheduled fight, which co-headlines UFC on FOX 5.

Penn (16-8-2 MMA, 12-7-2 UFC), who returns to the cage after retiring 13 months ago, has MacDonald (13-1 MMA, 4-1 UFC) to thank for his transformation from unmotivated couch potato to elite athlete.

The former UFC lightweight and welterweight champion, whose legacy has in part been defined by such changeovers, said he planned to “get into shape a little” and give it his best effort against MacDonald, who’s widely been characterized as a future champion at 170 pounds.

But when the 23-year-old training partner of welterweight champ and former nemesis Georges St-Pierre made some disparaging tweets about his physique, Penn’s mindset changed.

“That really lit a fire under my butt,” he said.

Penn rose from the couch and hired the trainer that guided him to standout performances against Jens Pulver, Joe Stevenson and Sean Sherk.

“We took my belly off,” he said. “I think the best thing for me was the fight getting postponed.”

The pair’s originally scheduled meeting was pushed back when MacDonald sustained a cut in training that forced him to withdraw from UFC 152. The bout was then slotted to co-headline Dec. 8’s UFC on FOX 5, which takes place at KeyArena in Seattle. The event’s main card airs live on FOX while prelims serve on FX and Facebook.

Penn never stopped training in the interim and hosted a bevy of top fighters in his Hilo, Hawaii, gym including Strikeforce fighters Tyron Woodley and Pat Healy.

Along with his harsh tweets, MacDonald’s reputation as a future champ pushed Penn to excel in camp.

“I realized I can’t do this forever, so I might as well make the most of it,” he said. “Everybody says he’s going to be a champion soon, so ending up fighting somebody like Rory, it wasn’t a tough decision.”

When on point, Penn has been called one of the greatest to compete in the sport. When bored, he’s disappointed not only fans but UFC brass. He and UFC President Dana White have frequently butted heads over the course of his career.

Today during a conference call, however, Penn called White “the man” and said the UFC executive was an expert at putting together fights that fans wanted to see – namely, a superfight between St-Pierre and middleweight champ Anderson Silva.

White, in turn, praised the 33-year-old fighter and said MacDonald had a tough challenge ahead.

“The one thing you always know with B.J. Penn – you’re not going in there and running over B.J. Penn,” White said. “B.J. is going to fight until the end. He’s never been cut, he’s never been knocked out, and B.J.’s a top guy.”

Penn admitted he lost the drive to prepare properly for fights. Following a loss to Nick Diaz at UFC 137, he retired and later said he saw no challenges to motivate him.

But today, he said his fighting spirit never left, and a ticking clock urged him to beat back young contender MacDonald.

“Fighting is still not a sport for me,” he said. “I don’t know if I should be saying that on a conference call. Fighting is still a fight for me. It always has been. I’m not a great athlete that can play any sport. But one thing I could always do is … fight back.

“This is a fight. This isn’t putting a ball in a hole. This has always been a fight for me, and I just love it when it’s that way. I love it when my opponent says that: He’s fat, I’ll end up killing him, he’s nothing, this and that. It’s all wonderful. I couldn’t ask for anything more.”

In a subtle dig at St-Pierre, Penn, who lost a bid for the Canadian’s belt in a superfight at UFC 94, talked up his spirit to take on bigger challenges.

“You know what I would do if Dana called me and said to fight the champion above,” he said. “You know what I would do.”

As the UFC 189 tour made its last stop in Dublin, featherweight champ Jose Aldo was met with a torrent of abuse from the Irish fans. It might have been unpleasant, but it might also have been just what he needed.